1. Field of the Invention
The present invention relates to a gas sensor.
2. Description of the Related Art
Hitherto, there is known a gas sensor for detecting a concentration of specific gas, such as NOx, which is contained in target gas of measurement, e.g., exhaust gas from an automobile. For example, Patent Literature (PTL) 1 discloses a gas sensor including a sensor element that detects a concentration of specific gas, a tubular member through which the sensor element penetrates, and a cover disposed at a free end of the sensor element and having a gas inlet through which the specific gas is introduced.
Furthermore, PTL 2 discloses a gas sensor including a ceramic holder (tubular member) having a recess and a penetration hole formed in a bottom portion of the recess, a gas sensor element inserted into both the recess and the penetration hole, and a sealing material (molten glass powder) filled into a void space between the gas sensor element and the recess. PTL 2 states that, in the disclosed gas sensor, a bonding failure between an inner peripheral surface of the recess of the ceramic holder and the sealing material can be suppressed and a good gas-tight state can be obtained by setting surface roughness of the inner peripheral surface of the recess of the ceramic holder to be not less than 0.3 μm, and by setting surface roughness of the bottom portion of the recess to be larger than that of the inner peripheral surface of the recess. PTL 2 further states that the surface roughness of the inner peripheral surface of the recess of the ceramic holder is preferably not less than 2 μm and more preferably not less than 5 μm.